Cam ring motors, i.e. radial piston hydraulic motors provided with a cam ring, have been known for quite some time. In such a motor, radial pistons are provided with rollers that are pressed against a wave-shaped inner surface of the cam ring. It is characteristic of the motor that a specific supply pressure must be constantly available so that the rollers of the pistons stay engaged to the wave-shaped cam ring. As the rotating speed of the motor rises, the output, i.e. volume flow, supplied by the pump is at some point no longer sufficient relative to the speed of the motor, in which case, without special arrangements, the rollers of the pistons start to disengage from the cam ring, the hydraulic motor starts to sound abnormal and is obviously at risk from breakage. In this situation, the motor must be couplable into freewheeling and, in addition, the motor must be structurally and functionally such that the pistons, especially the rollers of the pistons, automatically come off the cam ring. To this end, the motor must be provided with a special freewheeling valve. As one example of such a solution, a radial piston hydraulic motor and a method in the control thereof as described in FI patent publication No 118233 are disclosed.
Other state of the art solutions are also known. As one example, an arrangement described in U.S. Pat. No. 5,224,411 is disclosed, wherein two hydraulic motors are supplied by a hydraulic pump, one being continuously coupled to one pump and the other being disengageable from it. The motor that is disengageable is of a type where the pistons come off the cam ring when pressure is not supplied to the motor. The system comprises a check valve disposed in a distributor valve for preventing the cams from being struck against the cam ring when the cam ring starts to press them into the group so as to assume a freewheeling position. This is effected so that the check valve prevents the oil that has been discharged from under the pistons from flowing under the pistons that are already pressed into the block and lifting them back up. The only way for the oil is to the tank line. However, the control valve described in the patent referred to above does not function automatically but requires that the freewheeling position be manually switched on.
With respect to the state of the art, reference is also made to U.S. Pat. No. 6,508,328 describing a hydraulically operated working machine driving system. A by-pass valve is provided in connection, with the motor and disposed in a block external to the motor for preventing the hydraulic motor from cavitating and unnecessarily braking in a situation where the speed of the machine is high and the wheels rotate faster than the pump outputs oil. The machine has a mechanical main power transmission, so this possible. In this case, the valve connects lines A and B of the motor to each other so that the supply pressure acts under the pistons and the rollers of the piston follow the cam ring. When the speed of the wheels slows down to a degree that the output of the pump is sufficient again, this valve automatically couples the by-pass flow to the plug, a full supply pressure is conveyed to the pistons and the motor starts to drive again. However, the valve does not couple the pistons into freewheeling, i.e. into the cylinder block off the cam ring. The pistons continuously hold contact to the cam ring, and no real freewheeling is established. As the pistons follow the cam ring, power losses occur.